Arc-boutant

English: In architecture, a flying buttress, or arc-boutant,is usually on a religious building, used to transmit the thrust of a vault across an intervening space (which might be an aisle, chapel or cloister), to a buttress outside the building. The employment of the flying buttress means that the load bearing walls can contain cut-outs, such as for large windows, that would otherwise seriously weaken the vault walls.